Device for tuning a Bragg grating by means of compression using a piezoelectric actuator

ABSTRACT

A device for tuning a Bragg grating, by compression by a piezoelectric actuator. The device is particularly applicable to optical telecommunications and for example includes a mechanism to compress an optical fiber portion, containing a Bragg grating, and a mechanism to prevent buckling of the portion, a tube crossed by the portion, and a guide to guide this portion in the tube. The compression mechanism includes a curved deformable component and a piezoelectric actuator that is positioned between the component and the tube, and that extends when it is energized and thereby deforms the component, the latter then compressing the portion.

TECHNICAL FIELD

The present invention relates to a device for tuning a Bragg grating bycompressing the latter.

It is particularly applicable to optical telecommunications, and morespecifically, to those which implement DWDM or Dense Wavelength DivisionMultiplexing.

With the invention, for example, it is possible to form filters,routers, or add-drop devices according to the wavelengths of incidentlight signals.

STATE OF THE PRIOR ART

It is known how to form a Bragg grating in an optical fiber and to usethis grating as a wavelength tunable component.

To tune this grating, it is known how to longitudinally deform theoptical fiber in which it is found; so as to change the pitch of thegrating and therefore the wavelength response of the latter.

Devices for tuning a Bragg grating formed in an optical fiber bycompression of the latter along its longitudinal axis, while preventingbuckling of the fiber, are already known.

On this matter, reference will be made to the following documents:

[1] U.S. Pat. No. 5,469,520 “Compression-tuned fiber grating”

[2] WO 00/37969 “Compression-tuned Bragg grating and laser”.

DISCUSSION OF THE INVENTION

The object of the present invention is a device for tuning a Bragggrating which may be made very compact.

This device includes a piezoelectric actuator in order to compress theoptical fiber, in which the grating is formed, and means for preventingbuckling of the thereby compressed fiber.

Preferably, the device further comprises means for controllingcompression of the fiber and position-locking means.

The relative simplicity and the compactness of the device, object of theinvention, guarantee good integrability and a low manufacturing cost.

Moreover, the compactness of the device gives it low mechanical inertiaand therefore a short response time.

Specifically, the object of the invention is a device for tuning areflector component formed in a portion of an optical waveguideincluding first and second ends, this optical waveguide being intendedfor propagating a light, the reflector component being capable ofreflecting this light at a reflection wavelength, this device comprising

-   -   means for compressing the portion of the optical waveguide and        therefore the reflector component so as to change the reflection        wavelength, and    -   prevention means, for preventing buckling of the optical        waveguide portion when the latter is compressed,

this device being characterized in that the prevention means comprise

-   -   a tube with first and second ends, this tube being crossed by        the optical waveguide portion, and    -   means for guiding this portion in the tube,

and the compression means comprise

-   -   a curved deformable component, with first and second sides, the        first respective ends of the tube and of the optical waveguide        portion being attached to the first side, the second end of the        tube being spaced apart from the second side and the second end        of the optical waveguide portion being attached to this second        side, and    -   a piezoelectric actuator positioned in a space between the        curved deformable component and the tube and attached to this        component and to this tube, this actuator being capable of        extending when it is energized and then deforming the component,        the latter being then capable of compressing the optical        waveguide portion.

Preferably, the reflector component is a Bragg grating.

Further, the optical waveguide is preferably an optical fiber.

According to a preferred embodiment of the device object of theinvention, the compression means have an axis of symmetry which isformed by the axis of the optical waveguide portion.

According to a first particular embodiment of the device, object of theinvention, the guiding means comprise rings which extend one after theother in the tube, are spaced apart from one another by elasticcomponents, preferably elastic toric spacers, and crossed by the opticalwaveguide portion, this optical waveguide portion being capable offreely sliding in these rings.

These elastic components preferably are in honeycombedpolytetrafluoroethylene.

According to a second particular embodiment of the device, object of theinvention, the guiding means comprise stiff washers which are placed oneafter the other in the tube, along the optical waveguide portion, andcrossed by this optical waveguide portion, and elastic components whichextend one after the other in the tube, alternate with the stiff washersand are integral with these stiff washers.

Preferably, the elastic components form a unique block of elasticmaterial which confines the optical waveguide portion.

According to a preferred embodiment, the device, object of theinvention, further comprises means for controlling the piezoelectricactuator in a closed loop configuration.

These control means may comprise measuring means comprising the Bragggrating or a variable capacitor with two plates which are integral withthe tube and the deformable component, respectively.

The device, object of the invention, may further comprise means forblocking the deformable component.

Preferably, these blocking means comprise a component which is made outof a shape memory alloy and capable of tightening the tube.

SHORT DESCRIPTION OF THE DRAWINGS

The present invention will be better understood upon reading thedescription of exemplary embodiments given hereafter, in a purelyindicative and by no means limiting way, with reference to the appendeddrawings wherein:

FIG. 1 is a schematic view of a first particular embodiment of thedevice, object of the invention,

FIG. 2 is a schematic and partial view of the device of FIG. 1, and

FIG. 3 is a schematic and partial view of a second particular embodimentof the device, object of the invention.

DETAILED DISCUSSION OF PARTICULAR EMBODIMENTS

The device according to the invention, which is schematicallyillustrated in FIG. 1, is intended for tuning a Bragg grating 2 which isformed in a portion 4 of an optical fiber 6. To tune this grating, thedevice compresses the portion 4 of the optical fiber, which isrectilinear. The axis of this optical fiber portion, which is also theaxis along which this portion is compressed, is marked as X.

The device of FIG. 1 comprises means 8 for compressing the optical fiberportion as well as means 10 provided for preventing buckling of thisoptical fiber portion when it is compressed longitudinally. These means10 provided for preventing buckling are illustrated in more detail inFIG. 2.

In the example of FIG. 1, the means 8 for compressing this optical fiberportion are symmetrical relatively to axis X and comprise a deformablecomponent 12, with a substantially elliptical shape, which is made outof a polymer material, for example.

In FIG. 1, a stiff tube 14 is also seen, the axis of which is axis X. Anend of this tube 14 is attached to the internal wall of a part 16 ofcomponent 12 (lower part in FIG. 1).

The other end of the tube 14 faces the internal wall of the other part18 of component 12 (upper part in FIG. 1) and is spaced apart from thispart 18.

The compression means 8 also comprise a double piezoelectric actuatorincluding two piezoelectric components 20 and 22, with an elongatedshape, which are positioned in the space delimited by component 12.Moreover, in this space, components 20 and 22 extend perpendicularly toaxis X.

In the example of FIG. 1, the tube 14 extends along the minor axis ofthe substantially elliptical component 12 whereas the piezoelectriccomponents 20 and 22 extend along the major axis of this component 12.

Moreover, the piezoelectric component 20 is attached, on one side, totube 14, and on the other side to component 12. Likewise, thepiezoelectric component 22 is attached on one side, to tube 14 (oppositecomponent 20) and on the other side to component 12.

A guiding tubular component 24, the axis of which is axis X, is alsofound in the space delimited by component 12 and this tubular componentis attached to the internal wall of part 18 of this component 12.

The end of the tube 14, which is found facing this part 18, is placed inthe tubular component 24 and is slightly spaced apart from it in orderto be able to slide within this tubular component upon compression ofthe optical fiber portion, as it will be seen later on.

Means 26 for controlling the piezoelectric components 20 and 22 are seenin FIG. 2. When these components 20 and 22 are energized by thesecontrol means 26, components 20 and 22 extend perpendicularly to the Xaxis (arrows F1 and F2 in FIG. 1), component 12 is deformed, and tube 14as well as the tubular component 24 move relatively to each other(arrows F3 and F4 of FIG. 1), which increases the overlapping of the endof the tube 14 with this tubular component 24.

It is specified that an end of the optical fiber portion 4 is attachedto part 16 of the component 12 whereas the other end of this opticalfiber portion is attached to the other part 18 of this component 12.

More specifically, it is seen that these parts 16 and 18 comprise bores28 and 30 at the X axis, respectively. The optical fiber portion isattached in each of these bores. Deformation of component 12, which ismentioned earlier, thereby causes compression of the optical fiberportion.

If a light I emitted by a large bandwidth light source (not shown) isinjected into the optical fiber 6, this light arrives on the Bragggrating 2. The latter reflects the light R with a wavelength whichdepends on the pitch of the grating. The optical fiber transmits thelight T which is not reflected by this grating.

When the optical fiber portion 4 is compressed, the pitch of the gratingis reduced and the wavelength of the reflected light R is changed.

In the example of FIGS. 1 and 2, the means 10 which prevent buckling ofthe optical fiber portion during its compression, comprise rings 32 andelastic components 34.

These components 34 are toric spacers which are made out of an elasticmaterial with low friction coefficients, preferably honeycombedpolytetrafluoroethylene.

The rings 32 are placed one after the other in tube 14. These ringsencircle the optical fiber portion 4 and are spaced apart from oneanother by means of elastic toric spacers 34.

Each toric spacer 34 allows two adjacent rings 32 to be spaced apart bypressing on two chamfers 36, at 45°, formed on the ends of these rings,respectively, which are facing each other.

Thus, the optical fiber portion 4 is guided in all the rings, the latterbeing held longitudinally by the tube 14 which limits axial offset ofthese rings, for example to within ±0.5 μm.

With the small axial and especially longitudinal play which is evenlydistributed among the rings by all the toric end pieces or gaskets 34,buckling of the optical fiber portion 4 may be avoided during itscompression.

Moreover, the rings are self-aligned regardless of the longitudinaldisplacement imposed by the substantially elliptical component 12, dueto the presence of the chamfers 36 at 45° pressing onto the toricgaskets 34 symmetrically.

It should be noted that the nearest ring to part 16 of this component 12may be attached to this part 16 and the nearest ring to part 18 of thecomponent 12 may be attached to this part 18, but this is not mandatory.

In a known way, a temperature-compensating Bragg grating 36 differentfrom grating 2, may be provided in a portion of the optical fiber 6which is not subject to compression, for example in the portion of thisfiber which is found in the bore 28, where the optical fiber portion 4is attached.

It should be noted that the deformation induced by the doublepiezoelectric actuator is amplified by the substantially ellipticalcomponent 12. An open loop configuration may be used for controllingthis piezoelectric actuator.

However, a closed loop configuration is preferably used (FIG. 2). To dothis, the longitudinal deformation of the piezoelectric components 20and 22 may be measured by means of a variable capacitor, the plates ofwhich 38 and 40 are coaxial.

One of these plates, with reference 38, results from metallizing theexternal wall of the end of tube 14 which is capable of sliding withinthe tubular component 24. The other plate 40 results from metallizingthe internal wall of this component 24.

In this case, the tube 14 and the component 24 are electricalinsulators, for example made out of a stiff plastic material.

The capacitance of the variable capacitor 38–40 is a linear function ofthe position of the end of the tube 14 relatively to the component 24and therefore of the longitudinal deformation of the piezoelectriccomponents 20 and 22.

The plates 38 and 40 are electrically connected to the control means 26and provide the latter with the information relative to thislongitudinal deformation.

As an alternative, in order to measure this longitudinal deformation, aportion 42 of the light R reflected by the Bragg grating 2 is recovered,for example via an optical coupler (not shown) which is inserted intothe optical fiber 6, on the outside of the device of FIG. 1, and thislight is processed by a suitable photo-detection interface 44 which thenprovides the control means 26 with the information relative to thelongitudinal deformation.

Means 46 for immobilizing the device in any position, corresponding to adetermined extension of the piezoelectric components 20 and 22 arefurther provided in the example of FIG. 2.

These means 46 comprise a ring or a spring 48 which is made out of ashape memory alloy. This ring or spring is attached to the end of thetubular component 24 on the side of the internal wall of the latter.

In the example of FIG. 2, the immobilizing means also comprise anotherring 50 forming a braking ring, which is between the ring 48 and the endof tube 14.

As is seen in FIG. 2, the end of the tubular component 24 comprises ashoulder which supports the ring 48 and the braking ring 50.

Actuation of the blocking allowed by the ring 48 is performed byreducing the internal diameter of this ring at room temperature.

Expansion of the ring 48 which allows free movement of the tubularcomponent 24 actuated by the piezoelectric components 20 and 22, occursunder the effect of a rise in temperature which is induced by the Jouleeffect, which causes the phase transition of the shape memory alloy.

In order to raise the temperature of the ring 48, the latter isconnected to heating control means 52 (voltage source) via electricalconnections 54.

The “trained” diameter reduction is reversible by returning to a statewhich is also “trained” corresponding to the other phase, i.e., theexpanded phase, of the shape memory alloy.

The diameter reduction tightens the braking ring 50 which then locks thetubular component 24 on the tube 14.

In another example not requiring the training of both phases of theshape memory alloy, instead of the set of rings 48 and 50, the couplingof a shape memory alloy (SMA) associated with a cladding acting as aspring and made for example from a polymer, is used. With the differentvalues of Young's modulus of the SMA in the martensitic and austeniticphases, the ring-spring, for which the product of Young's modulus by thesection is intermediate to that of the SMA ring, may control the“expanded” or “closed” state of the blocking device.

In the example of FIG. 3, the means 10 for preventing the buckling ofthe optical fiber portion 4 comprise stiff washers 56 which arepositioned in the tube 14, parallel to each other and perpendicularly tothe X axis, and which encircle the optical fiber portion. These washersare spaced apart from each other by elastic components 58.

These elastic components 58 only form a single block which is made outof elastomeric material, confines the optical fiber portion 4 andextends from part 16 to part 18 of the component 12 as is shown in FIG.3.

The stiff washers are slightly spaced apart from the internal wall ofthe tube 14.

When the fiber portion 4 is compressed, the washers move along the Xaxis, while being guided by the tube 14.

It is specified that the one-piece assembly of components 58 may be madeby moulding and injecting an elastomer in a single part which integratesthe optical fiber portion 4.

Moreover, the washers are machined with a precision of ±0.5 μm in orderto limit axial offset. These washers are made integral with theelastomer by adhesion during the moulding; the same applies for thefiber portion.

In the example of FIG. 3, blocking means 46 may again be used andelectrical connections and control means (not shown), mentioned in thedescription of FIG. 2, may be added to them.

A closed loop feedback control may also be used for the piezoelectriccomponents 20 and 22 and the device may be provided with the coaxialcapacitor (not shown) which was mentioned in the description of FIG. 2.

1. A device for tuning a reflector component formed in a portion of anoptical waveguide including first and second ends, the optical waveguideconfigured to propagate a light, the reflector component being capableof reflecting the light at a reflection wavelength, the devicecomprising: means for compressing the optical waveguide portion andtherefore the reflector component, to change a reflection wavelength;and prevention means for preventing buckling of the optical waveguideportion when the optical waveguide portion is compressed, wherein theprevention means comprises: a tube with first and second ends, the tubebeing crossed by the optical waveguide portion, and means for guidingthe portion in the tube; wherein the means for compressing comprises: acurved deformable component, with first and second sides, firstrespective ends of the tube and the optical waveguide portion beingattached to the first side, a second end of the tube being spaced apartfrom the second side and the second end of the optical waveguide portionbeing attached to the second side, and a piezoelectric actuator,positioned in a space between the curved deformable component and thetube, and attached to the component and to the tube, the actuatorconfigured to extend when energized and then deforming the component,the latter being then configured to compress the optical waveguideportion.
 2. The device according to claim 1, wherein the reflectorcomponent is a Bragg grating.
 3. The device according to claim 1,wherein the optical waveguide is an optical fiber.
 4. The deviceaccording to claim 1, wherein the compression means has an axis ofsymmetry that is formed by the axis of an optical waveguide portion. 5.The device according to claim 1, wherein the guiding means comprisesrings that extend one after the other in the tube, are spaced apart fromone another by elastic components and/or elastic toric spacers, andcrossed by the optical waveguide portion, the optical waveguide portionconfigured to freely slide in the rings.
 6. The device according toclaim 5, wherein the elastic components are in honeycombedpolytetrafluoroethylene.
 7. The device according to claim 1, wherein theguiding means comprises stiff washers placed one after the other in thetube, along the optical waveguide portion, and are crossed by theoptical waveguide portion, and elastic components that extend one afterthe other in the tube, alternate with the stiff washers and are integralwith the stiff washers.
 8. The device according to claim 7, wherein theelastic components form a single block of elastic material that confinesthe optical waveguide portion.
 9. The device according to claim 1,further comprising means for controlling the piezoelectric actuator in aclosed loop configuration.
 10. The device according to claim 9, whereinthe control means comprises measuring means comprising the Bragg gratingor a variable capacitor with two plates that are integral with the tubeand the deformable component, respectively.
 11. The device according toclaim 1, further comprising means for blocking the deformable component.12. The device according to claim 11, wherein the means for blockingcomprises a component that is made out of a shape memory alloy andconfigured to tighten the tube.